Who is Biff (Levi) from Lamb: The Gospel According to Biff and what is their importance?

Biff is the childhood pal of Joshua, later known as Jesus Christ. He grows up with Joshua in Nazareth and knows of his special station in life. Always quick with a wisecrack, Biff writes his gospel about the incidents leading up to Joshua's ministry that leads to his crucifixion. Biff tells about their mutual love for Maggie, Joshua's use of his powers to get out of jams, and the travels to the East where Joshua learns how to be the Messiah.